Flexible Light‐to‐Frequency Conversion Circuits Built with Si‐Based Frequency‐to‐Digital Converters via Complementary Photosensitive Ring Oscillators with p‐Type SWNT and n‐Type a‐IGZO Thin Film Transistors

Abstract In this study, as system‐level photodetectors, light‐to‐frequency conversion circuits (LFCs) are realized by i) photosensitive ring oscillators (ROs) composed of amorphous indium‐gallium‐zinc‐oxide/single‐walled carbon nanotube (a‐IGZO/SWNT) thin film transistors (TFTs) and ii) phase‐locked‐loop Si circuits built with frequency‐to‐digital converters (PFDC). The 3‐stage ROs and logic gates based on a‐IGZO/SWNT TFTs successfully demonstrate its performance on flexible substrates. Herein, along with the advantage of scalability, a‐IGZO films are used as photosensitive n‐type TFTs and SWNTs are employed as photo‐insensitive p‐type TFTs for better photosensitivity in circuit level. Through the controlling a post‐annealing condition of a‐IGZO film, responsivities and detectivities of a‐IGZO TFTs are obtained as 36 AW⁻¹ and 0.3 × 10¹² Jones for red, 93 AW⁻¹ and 3.1 × 10¹² Jones for green, and 194 AW^–1 and 11.7 × 10¹² Jones for blue. Furthermore, as an advanced demonstration for practical application of LFCs, a unique circuit (i.e., PFDC) is designed to analyze the generated oscillation frequency (f_osc) from the LFC device and convert it to a digital code. As a result, the designed PFDC can exactly count the generated f_osc from the flexible a‐IGZO/SWNT ROs under light illumination with an outstanding sensitivity and assign input frequencies to respective digital code.